This invention relates to water purification and, more particularly, this invention relates to reverse osmosis systems for water purification. Specifically, this invention is concerned with a method and composition for inhibiting scale in reverse osmosis water purification systems.
One of the most abundant natural resources on this planet, earth yet, paradoxically, one of the scarcest natural resources, is water. While fully seventy-five percent (75%) of the surface of the planet is water, actually very little of it is available for use by man without further treatment since most of it is saline or brackish. Furthermore, the distribution of water, especially potable water, on the planet is such that many inhabited areas are not blessed with an abundance of potable water. Thus, there is a need for purifying saline or brackish water to obtain potable water in many parts of the world.
There are other, man-made, environments wherein there is a great need for efficient and relatively inexpensive means for purifying saline or brackish water. For example, off shore drilling rigs are literally surrounded by water, none of it fit to drink and transporting water to such locations by tanker is prohibitive in cost. One of the most efficient means devised for providing fresh water for such environments is reverse osmosis water purification systems.
The principle of reverse osmosis has been known for many years. In its simplest form, osmosis utilizes a membrane which is semi-permeable to water but which rejects certain dissolved salts. Under normal conditions, if pure water is separated from a salt solution by a semi-permeable membrane, the water will flow through the membrane from the pure water side to the impure water side that is, from a dilute solution to a more concentrated solution, thereby diluting the more concentrated solution and continuing until osmotic equilibrium is reached. Osmotic equilibrium is reached when the osmotic head equals the osmotic pressure of the salt solution. It is well-known, however, that if a positive pressure is applied to the salt solution in an amount sufficient to overcome the osmotic pressure, the flow will be reversed and water will flow from the salt solution through the membrane to the pure water side of the membrane. Hence the term reverse osmosis.
In a reverse osmosis water purification system, impure water is pumped under high pressure into the system were it contacts the semi-permeable membrane. The product water which is 95-99% free of disolved minerals emerges from the membrane and passes out of the apparatus. Since the output of a reverse osmosis purification system relies to a great extent on the surface area of the membrane, there have been many developments in the improvement of systems which maximize the membrane area while packaging the membrane in a minimum of space. A typical structure used in making a reverse osmosis water purification device is shown in an oversimplified form partially schematically and partially in cross-section in FIG. 1. The structure, generally designated by the numeral 10, comprises membrane 12 supported on a cellular polymer matrix 14 which, in turn, is supported on a central foraminous support 16. The foraminous support, or screen, 16 essentially acts as a spacer as well as providing some mechanical support. Spongy polymer matrix 14 is any suitable open-celled foam material which will provide some mechanical support as well as providing free passage for the water. The membrane 12 is a thin, somewhat delicate, semi-permeable membrane made of any suitable material such as cellulose acetate, cellulose triacetate, a polyimide, or a polysulfone. If this structure is surrounded by saline water under pressure, at a pressure typically 600 to 800 psi, the water will pass through the membrane, travel through the polymer matrix, and then through the screen and emerge at the outlet 18 at one end of the structure. Essentially, the water follows the path designated by the arrows in FIG. 1. The membrane rejects the dissolved mineral salts. The end opposite the outlet 18 is closed.
Considering the structure shown in FIG. 1, it is clear that a structure large enough to provide a commercially usable flow of potable water will be extremely large and impractical. Thus, this structure, for use in modern equipment, is spirally wound in jelly-roll fashion and inserted in a rigid casing to provide a replaceable cartridge for the equipment. The water enters the cartridge through an inlet and flows through the spiral windings, with water passing through the membrane throughout the cartridge and emerging through a pipe at the center of the spiral. The now concentrated saline water passes out through an outlet.
It is to be distinctly understood that this invention is not at all related to the reverse osmosis water purification equipment, this brief simplified description of the typical apparatus being presented only for the purpose of understanding the system with which we are concerned and the problems which are to be solved.
Continuing with a description of the background of the invention, a typical water purification system based on reverse osmosis is shown schematically in FIG. 2. The impure water enters the pump 20 at 22. Certain chemical additives whose purpose will be described later are injected into the impure water flow at 24 and are thoroughly mixed in the impure water by a mixer 26. The water passes through filter 28 to remove solid impurities and then through booster pump 30 to a cartridge filter 32 where smaller solid impurities are removed. The water is then pumped by pressure pump 34 to the reverse osmosis apparatus 36. The water which passes through the reverse osmosis membrane in a cartridge passes out of apparatus 36 at outlet 38. Disinfectant is added at 40 and the final product passes out of the system at 42. The concentrated brine emerges from the system at 44.
The continued efficiency of a reverse osmosis system depends on the maintenance of the membrane in an unfouled condition. Probably the greatest problem experienced in the use of these systems is fouling of the membrane by scale. Typically, the membrane becomes fouled by scale build up to a point where it must be replaced quite often, sometimes as often as several times a month. The cartridge must then be removed and replaced by a clean cartridge. The used cartridge is then treated to remove scale. Obviously, it is desirable to prevent scale build-up, or, at least, prolong the time between cartridge changes. This is ordinarily done by injecting certain chemical additives to the impure water, these additives being used for the purpose of preventing the build-up of scale.
In the prior art, "scale" generally refers to calcium and magnesium scale.
There are a number of known additives for preventing scale build-up. Hexametaphosphate is widely used, as is sulfuric acid, in an amount sufficient to lower the pH to about 4-5. Without going into great detail in describing the mechanism of the prevention of scale build-up, it is known that hexametaphosphate prevents the growth of a precipitate of calcium magnesium oxide hydroxide. Sulfuric acid raises the solubility of calcium and magnesium salts thereby resulting in less of a precipitate. Both of these prior art additives retard scale build-up but do not stop it altogether. It is still necessary to remove the membrane and clean it at least once a month.
Another material commonly used as a scale inhibitor is a composition of polyacrylic acid having a molecular weight of about 20,000, chlorine, and calcium hypochlorite. The chlorine and calcium hypochlorite are to inhibit microbial growth. The major disadvantage of this composition is that the chlorine damages certain membranes to the point where they must be replaced and discarded.
The use of polyacrylic acid having a molecular weight in the range of about 20,000 to 22,000 is known in the prior art as a scale inhibitor. But, it is still quite inefficient.
Still other materials for use as scale inhibitors have been investigated, but have not found wide acceptance due to cost or other factors. For instance, in a proposal made by George H. Nancollas of the State University of New York at Buffalo to the National Science Foundation (proposal No. 67850540), polyphosphates, polycarboxylates, and polyphosphonates were described as well as low molecular weight polyacrylates. While the proposal does not specify the molecular weight of the polyacrylates, a material known as Calnox 214DM is the material used.
The work done by Nancollas was concerned with scale in heat exchange systems and the like and was not concerned in any way with reverse osmosis waer purification systems. The tests were all run under laboratory conditions using artificially constituted solutions of calcium sulfate dihydrate. Magnesium salts were also considered. Other materials tested were diethylenetriaminepenta-(methylene phosphonic acid) and phytic acid. There was also the suggestion that synergistic mixtures of phytic acid and phosphonates or phytic acid and diethylenetriaminepenta-(methylene phosphonic acid).
The work done by Nancollas, however, is not at all concerned with reverse osmosis systems and, in fact, there is no suggestion that his work could be extrapolated for use with reverse osmosis systems.
Furthermore, none of the prior art known to us mentions or even suggests iron scale as a problem with reverse osmosis systems. We have found, however, that iron scale is as great a problem as calcium and magnesium scale and, perhaps, an even greater problem.
The reason for this is that calcium and magnesium scale tend to grow on the membrane from which they must be removed. While they are ordinarily removed chemically, it is essentially a mechanical removal. On the other hand, iron tends to grow crystals in the membrane which not only clogs the pores but, as the crystals grow, causes actual physical damage to the membrane. Thus, the cleaning of the membrane of iron scale is more complex and delicate and must be done before the crystals grow large enough to damage the membrane. We have found that none of the known scale inhabitors are at all effective for the prevention of iron scale.